1. Field of the Invention
The invention pertains to the field of real image projection systems. More particularly, the invention pertains to a real image projection system having a tilted curved reflector, eliminating the need for a 50-50 beamsplitter in the system.
2. Description of Related Art
The present invention, referred to as an Ultra-High-Bright (hereinafter xe2x80x9cUHBxe2x80x9d) display system, pertains to a real image projection system and, in particular, to a system in which an image of a real object is formed in space, giving the illusion that a real object exists at that point in space, when in reality it does not.
Visual Display systems normally use a curved reflector with a beamsplitter positioned at a 45 degree angle to the curved mirror""s optical axis, to divert the input beampath at a 90 degree angle to the viewing axis or imaging beampath. This method has been used since the early 1950""s for flight simulation, and is referred to as the WAC window system. These systems typically are used in an on-axis configuration, meaning that the optical axis or the un-tilted curved mirror""s center of radius is located along the viewing axis. When viewing such an on-axis system, any object within the viewing area images within the system. When one looks straight at an un-tilted or on-axis curved reflector, the viewer sees an image of himself appearing upside down in the system. This generally is not a problem in flight simulators, since the cockpit typically is dark, and thus very little ghost imaging is visible.
One of the earliest working real image displays is depicted in White""s 1934 publication of xe2x80x9cFundamentals of Opticsxe2x80x9d. It shows a spherical mirror positioned behind a table. A flower vase is mounted below the table and a real image of the vase is projected sitting on the table-top. This system works, however, the viewer and room also are projected as a real image, which is known as ghosting. The White system uses an on-axis configuration, with the image being formed not at the focal point of the system or the center of radius of the curved mirror. This causes image distortion as the image deviates from the focal point of the system (center of radius).
In the late 1980""s, real image display systems were developed, consisting of two on-axis parabolic reflector segments, as shown in U.S. Pat. No. 4,802,750. This at least partially solved the ghost reflection problem, however, the manufacture of parabolic sectors was extremely difficult and the product was not commercially viable. In the early 1990""s, systems were built that used a beamsplitter with high reflection and low transmission to reduce the ghosting effects. This proved effective reducing the ghost image brightness from 21.5% to 5.4%, except it also reduced image transmission from 21.5% to 16.1%. Although the ghost image was much dimmer, it was still visible, and a serious distraction to the viewer.
In 1999, a system was developed using a circularly polarized window in an on-axis, WAC window style configuration, and the La Russa U.S. Pat. No. 6,163,408 was issued in 2000. This improvement effectively blocked all ghost reflections from the viewer, however the circular polarizer element had a transmission of 42%, which reduced the image brightness or transmission from 21.5% to 9%.
In 2000, a tilted system was developed, using an off-axis, curved reflector, as disclosed in International Patent Application No. PCT/US00/11234 and PCT Publication No. WO 00/65844. The system comprises a curved reflector tilted at an angle between 5 and 20 degrees from the viewing axis. A beamsplitter is positioned in the viewing axis, tilted at 45 degrees to the curved reflector axis. This is a significant improvement because it eliminates ghost imaging completely; the system no longer required a 42% transmission circular polarizer to eliminate ghosting. A 70% transmissive front window was instead used to provide a system transmission of 15%.
The goal of visual display companies has been to develop a system that has an image brightness and fields-of-view comparable to the parabolic system (e.g., U.S. Pat. No. 4,802,750), with no ghost reflections, and a system that can easily be manufactured.
The proposed UHB imaging system of the present invention incorporates a tilted mirror in conjunction with the classic single mirror imaging system depicted in White""s 1934 publication of xe2x80x9cFundamentals of Opticsxe2x80x9d. A preferred embodiment of the system of the present invention comprises a single curved reflector tilted approximately 15 degrees off-axis. The angle of tilt must be sufficient to allow the diverging beampath to strike the curved reflector without intersecting the area of the viewing window aperture. The angle of tilt preferably should be less than 17 degrees so as to minimize the effects of field curvature in the optical system. As the angle of tilt exceeds 17 degrees, the distortion caused by field curvature phenomena becomes critical. In one embodiment, light from a CRT or target object located directly below the curved reflector and facing forward, strikes a fold mirror or flat reflective surface positioned directly below the viewing window aperture with the reflective surface facing the target and curved reflector. Preferably, the fold mirror is positioned at an angle so that the light beam the monitor is reflected to the center of the curved reflector at an angle from horizontal equal to twice the tilt of the curved reflector. The diverging light beam reflects off the curved reflector and the converging beam exits on a horizontal axis, forming a real image at the focal point of the system. Because the system uses no beamsplitter, the system transmission is approximately 56% when used with a 70% Neutral Density Filter Window and 80% when used with no front window.
Now that a system transmission of 80% can be achieved, other devices can be incorporated into the real image projection system, that were not possible when system transmission was limited to 15-22%.
The present optical system exhibits more than a 300% increase in image brightness over prior art systems, and the elimination of all ghosting within the viewing area.
The present invention provides a method or optical configuration that has a high transmission or brightness, typically between 45% and 80% for the real image, with no visible ghost reflections, while maintaining a sufficient field of view or angle in which the image is visible. Another feature included in the configuration is a method of reducing the aberrations and distortions inherent in real image display systems.
A preferred embodiment of the invention is a tilted, off-axis, curved reflector of the conical family of curves, or an aspheric design. A toric curve optionally is used to reduce aberrations, inherent in off-axis systems. The curved reflector also optionally is a Mangin style reflector with two distinct curves, one on the concave and one on the convex for improved imagery. The UHB off-axis configuration can be applied to any optical imaging system incorporating a reflective imaging component that will form a real or virtual image or near-infinity image.
As an example, the optical axis of the curved mirror is tilted at a 15 degree angle to the viewing axis, with the top of the mirror leaning toward the front of the display. The target object or CRT screen is positioned under the curved reflector, facing the front of the display facing upward at 15 degrees. The diverging beampath of light from the target object or CRT strikes a vertical flat reflector or fold mirror at a 15 degree angle of incidence, reflecting at a complementary 15 degree angle and the reflected beam of light strikes the curved reflector with the axis or center of the reflected beam striking the physical center of the curved reflector. The diverging beam of light strikes the curved reflector at an angle of incidence of 15 degrees to the optical axis of the mirror. The light beam is then reflected as a converging beam of light off the curved mirror at a complementary angle of 15 degrees from the optical axis of the mirror, or along a horizontal line coincident with the viewing axis. The diverging beam of light emanating from the CRT or object and reflected off the fold mirror strikes the curved reflector at a 30 degree angle to the real image axis in relation to the vertex of the mirror surface or center of the mirror surface.
Outside light entering the window of the system, such as that reflected off the viewer or bright objects in the room, will be reflected downward by the tilted curved reflector and will not form an image in the viewing area. Therefore, visible ghost reflections are eliminated. To reduce the system size, a fold mirror optionally is positioned in the diverging beampath from the target object, to fold the beampath back into the display case. The object can then be moved to the new target focal point of the system inside of the case. The beampath length or focal length must be sufficient to allow the target object to be located outside of the beam of diverging light reflected off of the fold mirror to the curved reflector.
Optionally, a secondary target source is positioned behind the curved reflector, for example, when the curved reflector is a partially reflective beamsplitter coating, such as a 74% Reflective and 12% Transmissive coating. This allows the background image to be visible through the curved reflector. In an alternate configuration, a beamsplitter is positioned so that one edge of the beamsplitter is next to the top of the curved reflector, and the bottom edge of the beamsplitter is positioned at the bottom edge of the window aperture. The beamsplitter must allow the full converging beampath to pass through the beamsplitter, but not interfere with the diverging beampath from the target object. By using a high transmissive beamsplitter, such as an 83% transmissive and 12% Reflective coating, and positioning the background input target above the beamsplitter, a virtual image of the background target is formed floating in front of the curved reflector along a common viewing axis with the real image.
The system optionally is used with a window to isolate the inside of the display from the viewer. The window may be anti-reflective coated on one or both surfaces to reduce reflections. The window optionally is a tinted or neutral density filter, to reduce stray light from exiting the system, or it may be a circular polarizer.